Embodiments of the present specification relate generally to electric machines, and more particularly to a system and method for suppressing surface discharges on conductive windings of the electric machines.
Typically, an electric machine is representative of an electric motor that converts electric power to mechanical power or to an electric generator that converts mechanical power to electric power. In general, the electric machine includes a rotor, a stator, and windings. It may be noted that the windings are representative of electrically insulated conductors made into coils of many turns. The stator includes a plurality of radial slots in which the windings are positioned. In an example of the electric machine acting as the electric motor, electric current flows through these windings and produces an electric field that aids in rotating the rotor in the electric machine. As a result, the rotor produces mechanical power and provides this mechanical power to a load that is coupled to the electric machine.
In an aviation application, the electric machine may be operated at a higher altitude, for example 50,000 feet above the sea level, having low atmospheric pressure. However, at this low atmospheric pressure, the electric machine may have low air breakdown voltage, which causes surface discharges on the windings and may degrade insulation of the windings. Also, as the need for electric aircrafts and hybrid electric propulsion systems is increasing, high voltage devices are emerging, which requires the electric machine to be designed and operated at high voltages, such as +/−270V or +/−540V. However, operating the electric machine at such a high voltage and low atmospheric pressure may increase the electric field around the windings and may cause air breakdown in the electric machine. Moreover, if the electric machine is operated at a high voltage (e.g., +/−270V) and driven by power electronic converters such as insulated-gate bipolar transistor (IGBT) and silicon carbide (SiC) drives, a significant voltage shoots to the windings due to a fast rise time. As a consequence, surface discharge may occur on the windings at an exit of the radial slots of the stator and may degrade the insulation of the windings. This degradation of the insulation may in-turn cause failure of the electric machine.
In general, electric machines used in an aviation system are required to be light weight and have high-power density to save fuel in the system. Therefore, to reduce the weight of the electric machines, thin insulation is preferred around the windings in the electric machine. However, for high reliability and safety, the electric machines are required to be free from a partial discharge. In conventional medium voltage industrial line-fed electric machines, partial discharge resistant mica tape is used for insulating the windings. More specifically, the windings are wrapped with corona protection tape in the slots and stress grading tape outside the slots to minimize the electric field and prevent occurrence of surface discharges on the windings. However, for the electric machines driven by power electronic converters such as insulated-gate bipolar transistor (IGBT) and SiC drives, the electric field will be concentrated at the slot exit. The corona protection tape and the stress grading tape may not have sufficient thickness or layers to move this increased electric field away from the slot exit of the stator core, hence fail to prevent the occurrence of surface discharges on the windings. Moreover, if the thickness of these conductive tapes is increased, more heat will be generated around the windings, which in-turn damages the insulation of the windings.
Thus, there is a need for an improved system and method for suppressing surface discharges on the windings of the electric machine.